Telecommunication systems usually include transmitters/receivers that have to be connected to antennas to radiate and/or to receive an electromagnetic signal.
The RF connection between the transmitter/receiver and the antenna is made by an RF feed line that may be either of balanced type (for example in the most general case a coaxial cable) or of balanced type (most often a balanced two-wire line).
FIG. 1 shows an example of a prior balanced two-wire line including two electrical conductors 10, 12 insulated by respective sheaths 14, 16. The two electrical conductors are kept at a constant distance D from each other by an insulative wall 18 (or insulative spreaders) fastened to the sheaths of the conductors. The insulative wall 18 and the sheaths 14, 16 of the conductors are flexible so as to be able to bend the two-wire line during its installation. The various parameters of the line, notably the diameter of the conductors, the distance D between conductors, the thickness and the dielectric characteristics of the sheath and the spreaders, determine a given characteristic impedance Zc of the feed line.
Balanced two-wire lines have the advantage over unbalanced coaxial lines of having lower transmission losses per unit length and high characteristic impedances. Balanced two-wire lines have known and proven characteristics and properties in a gas, solid or hybrid medium the radio-frequency characteristics of which are very similar to those of air (relative dielectric and magnetic permitivity very close to 1), but nevertheless significantly modified by the presence of an environment near the line having a dielectric permitivity different from that of air, notably when that environment is moist or of a metallic nature.
In some configurations for feeding an HF antenna by a transmitter/receiver relatively far from the antenna the balanced two-wire line must be away from the ground to prevent modifications of its radio-frequency characteristics, which would lead to high losses through mismatching of the antenna with the transmitter or coupling with the ground.
FIG. 2 represents an HF transmitter/receiver feeding an antenna via an overhead balanced two-wire line.
In the FIG. 2 configuration, a transmitter/receiver 22 is in a semi-enclosed place 24 feeding, via a two-wire line 26, an antenna 28 outside that place and at a great distance from that place. The two-wire feed line 26 is held at a distance from the ground 30 by supports 32 fixed to the ground and at least the head of which supporting the two-wire line is in insulative material.
This type of overhead solution represented in FIG. 2 is not desirable for reasons of discretion in the case of military installations. On the other hand, the two-wire line 26 in FIG. 2 cannot be buried without seriously degrading its characteristics, leading to high HF signal transmission losses. This deterioration is mainly caused by the state and the nature of the backfilling soil which significantly modifies the characteristics of the line, possibly being moist and thus more or less conductive. The state of the soil may also vary as a function of prevailing climatic conditions.